Three-phase brushless DC motors have many uses, among which are as spindle motors for computer hard disk drivers, digital video disk (DVD) drivers, CD players, and tape-drives for video recorders. Such motors are recognized as having the highest torque and power capability for a given size and weight. Compared to DC motors employing brushes, brushless DC motors enjoy reduced noise generation and improved reliability because no brushes need to be replaced due to wear.
FIG. 1 shows such a three-phase brushless DC motor 10 with three phases A, B, C having three coils 12, 14, 16 connected to each other in a Y-configuration at a center tap 18. As is well-known, the coils 12, 14, 16 are part of a stator that causes a permanent magnet rotor to rotate. The three coils 12, 14, 16 are connected to the center tap 18 by respective resistors 19, 20, 21. The first coil 12 (phase A) is connected through a coil tap A to a supply voltage Vcc by a first high-side transistor 22 and to ground via a first low-side transistor 23; the second coil 14 (phase B) is connected through a coil tap B to the supply voltage Vcc by a second high-side transistor 24 and to ground via a second low-side transistor 26; and the third coil 16 (phase C) is connected through a coil tap C to the supply voltage Vcc by a third highside transistor 28 and to ground by a third low-side transistor 30. Each of the transistors is an NMOS transistor as is typical. Represented in FIG. 1 by voltage supply symbols are respective back EMF sources EA, EB, EC that are inherently induced by the permanent magnets of the rotor while the rotor is rotated.
This type of motor is driven by exciting its phases in a suitable sequence while always keeping two phases under power and leaving a third phase in tristate or floating with a high impedance (Z). For example, in FIG. 1 phase A is driven with a PWM signal (PWM in), phase B is coupled to ground, and phase C is left in tristate. This results in a current IA through the phase A having a value of +I, a current IB through the phase B having a value of -I, and zero current IC through the phase C. At predetermined instances the driving of the phases switches so that current is driven through the phase that was previously floating and one of the other phases is left floating such that the algebraic sum of the currents in the three phases are always equal to zero. The driving sequence is as follows where the first letter indicates the phase of positive current flow and the second letter indicates the phase of negative current flow: EQU AB--AC--BC--BA--CA--CB.
With respect to whichever phase is currently in tristate (phase C in FIG. 1), a BEMF is produced in that phase and can be measured by measuring the voltage difference between the coil tap for that phase (coil tap C in FIG. 1) and the center tap CT. When that BEMF changes its polarity with respect to the center tap voltage, a zero-crossing is detected. This provides an indication for the next phase advancement and the rate of zero-crossing occurrences is directly proportional to the motor speed. Consequently, BEMP zero-crossing information is important for speed control.
In recent systems with PWM control, the phase in tristate sees a common mode signal that varies between zero and Vcc/2 because of the two phases (ON/OFF) of the PWM signal. During the ON phase, the common mode is brought to Vcc/2 and during the OFF phase is brought to zero. The different impedances at the nodes also results in a differential voltage modulation across the phase in tristate and therefore a differential modulation of the BEMF.
Prior art PWM systems sample and measure the BEMF only during the ON phase and employ only a minimum mask to try to block the noises causes by the power commutations between the three phases of the motor. These prior art systems do not sample the BEMF during the OFF phase of the PWM signal, the phase in which the common mode is zero. This translates in an uncertainty in the measurement of the zero-crossing of the BEMF, and therefore, in a jitter in the synchronization of the phases of the motor.